Glazed substrates are widely used as thermal heads and printer heads for facsimile systems. As printing speeds have greatly increased in recent years, they are now used under very severe conditions such that the temperature rises and drops abruptly. In order to reduce the size of equipment and to lower production costs, an IC drive circuit has been formed on the glazed substrate, or a step of thick film technology has been used. For this purpose, it is required that the glazed layer have much higher heat resistance, e.g., to be free of any variations even after heat treatment at a temperature of at least 800.degree. C. for a period of about 30 minutes. In conventional glaze compositions, however, alkali oxides or alkali oxides and lead oxide are added thereto since great importance is placed on the smoothness of the glaze surface and/or ease of production. Thus, they usually can withstand only temperatures of at most 600.degree. C.
A so-called high temperature type of glaze substantially free of alkali oxides and lead oxide (which may contain alkali oxides and lead oxide as impurities) was recently developed. This type of glaze, however, can withstand only temperatures in the neighborhood of 700.degree. C. If the heat resistance of the glaze is attempted to be increased, problems arise in that surface smoothness is reduced and the coefficient of thermal expansion drops, resulting in an increase in the expansion difference between the glaze layer and the ceramic substrate.
If there is a large difference in the coefficient of thermal expansion between the glaze and the ceramic substrate, at the time of baking the glaze high stress is formed in the glazed layer. When such a glazed ceramic substrate is used as a thermal head, for example, in a facsimile system, microcracks, for example, are formed in the glazed layer since further thermal stress is applied to the glazed layer under severe conditions when the temperature rises and drops abruptly. Thus, the ultimate glazed ceramic substrate is poor in reliability. It is also known that if there is a large difference in expansion, cracks are formed even at the time of baking and cooling.